Opacity color

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

Css opacity background color

background: rgba(255, 255, 255, 0.25);

Background color with opacity

h1 {background-color:rgba(255,0,0,0.3);}

Background color opacity

rgba(51, 170, 51, .1)    /*  10% opaque green */ 
rgba(51, 170, 51, .4)    /*  40% opaque green */ 
rgba(51, 170, 51, .7)    /*  70% opaque green */ 
rgba(51, 170, 51,  1)    /* full opaque green */

Source: stackoverflow.com

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